Silent Afib May Raise Stroke Risk

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The presence of any event of subclinical atrial fibrillation in the first three months of the study more than doubled the annualized risk of stroke or peripheral emboli and increased the risk of clinically evident atrial fibrillation or flutter.

The study does not address the question of whether treatment with warfarin or other anticoagulants is indicated for the asymptomatic patient with brief episodes of atrial fibrillation.

Subclinical atrial fibrillation is relatively common among patients with pacemakers or implantable cardioverter-defibrillators (ICDs) and is associated with a higher risk of stroke or systemic embolism, researchers found.

In older patients with hypertension but no history of atrial fibrillation, 10.1% had a subclinical atrial tachyarrhythmia within three months of getting their implanted device, according to Stuart Connolly, MD, of McMaster University in Hamilton, Ontario, and colleagues.

Those who did were more than twice as likely to have an ischemic stroke or systemic embolism over two-and-a-half years of follow-up (HR 2.49, 95% CI 1.28 to 4.85), the researchers reported in the Jan. 12 issue of the New England Journal of Medicine.

The group originally reported these findings at the 2010 American Heart Association meeting.

In an accompanying editorial, Gervasio Lamas, MD, of the Columbia University Division of Cardiology at Mount Sinai in Miami Beach, Fla., said that the robustness of the data indicates that the association is true, adding that questions remain about the clinical significance.

"Until clinical trials targeting the population with short, asymptomatic episodes of high atrial rate are carried out, the current evidence simply does not address the question of whether treatment with warfarin or other anticoagulants is justifiable for the asymptomatic patient who has had a six-minute episode of atrial fibrillation," he wrote.

"For now, therefore, I will continue to turn to the now-venerable CHADS2 score, consider applying it to patients with asymptomatic episodes of atrial fibrillation lasting for hours, and make a clinical judgment about the need for anticoagulation."

About one-quarter of ischemic strokes are unexplained, and subclinical atrial fibrillation is believed to be a major culprit. Pacemakers can detect subclinical episodes of rapid atrial rate.

To examine the association between these subclinical atrial tachyarrhythmias and clinical outcomes, Connolly and colleagues started the ASSERT study, which included 2,580 patients 65 and older enrolled from 23 countries. All of the patients had hypertension, either a pacemaker (95%) or ICD (5%) placed recently, and no history of atrial fibrillation.

The researchers monitored the patients for three months with their implantable devices to detect subclinical atrial tachyarrhythmias, defined as episodes involving an atrial rate greater than 190 beats per minute for more than six minutes. Follow-up for clinical events lasted for an average of two-and-a-half years.

The presence of a subclinical atrial tachyarrhythmia within the first three months was associated with higher rates of clinical atrial fibrillation or flutter on a surface electrocardiogram (15.7% versus 3.1%; HR 5.56) and ischemic stroke or systemic embolism (4.2% versus 1.7%; HR 2.49) during follow-up.

Both differences were statistically significant (P≤0.007) and maintained significance after adjustment for other predictors of stroke.

Of the 51 patients who had an ischemic stroke or systemic embolism, 11 had a subclinical atrial tachyarrhythmia and none had clinical atrial fibrillation in the first three months.

The researchers found that the population attributable risk of stroke or systemic embolism related to subclinical atrial fibrillation was 13%, which is similar to the attributable risk of stroke associated with clinical atrial fibrillation found in the Framingham Heart Study.

A second objective in the study was to determine whether continuous atrial overdrive pacing influenced clinical outcomes. After randomizing the patients with pacemakers to having overdrive pacing turned on or off, the researchers found that it did not prevent atrial fibrillation or any other outcome. However, the study was underpowered to make firm conclusions about overdrive pacing outcomes.

Connolly and colleagues noted that only 15.7% of patients with subclinical atrial tachyarrhythmias developed clinical atrial fibrillation, "suggesting that there can be a lag between subclinical events and clinical detection."

Also, they advised that Holter monitoring for a few days may not detect subclinical atrial fibrillation as the median time that subclinical tachyarrhythmias were detected within the first three months was 36 days.

The investigators said that patients with pacemakers may have a higher prevalence of subclinical atrial tachyarrhythmias than other high-risk populations, suggesting the findings are not generalizable. However, previous studies have suggested that other elderly populations have a high prevalence of subclinical atrial fibrillation, they said.

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